It is a tool used to provide explanation. It sets a background for what is already there. It provides history. It is a tool meant to help the reader understand, to further the plot. Oftentimes it is a tool that does nothing but leave the reader absolutely confused. Flashbacks are extremely important to some stories, but quite often they’re not written properly.
Separate your flashbacks. This is a very simple thing to do, yet I always find writers who don’t. If your flashback isn’t separated from the text before or after it, do you know what readers are thinking? They’re thinking: Why the hell is this there? Where did this come from? Did I accidentally miss a chunk of text? That’s not to say they won’t catch on quickly, because they will, but they’re going to be frustrated and annoyed.
Italicize. Whether your flashback is set after a line break or whether it’s included in the main text, italicizing will let readers know the text is special. This is an especially useful tool if you want to do a flashback that is only a couple of sentences long. If you’re going to take the reader from the present to the past, you need to alert the reader. Not doing so makes it seem like you dropped a random chunk of text into the middle of your story. This disrupts the reading flow and confuses.
There’s a special kind of flashback, one that resembles a diary entry almost. The character isn’t so much recalling the memory as they are telling of the memory. The difference is that in one version the reader is transported to the actual scene, while in the other the character is more doing a, “I remember when…” The important thing to remember here is that you can’t just say ‘I remember when’ and then tell the scene like it’s taking place in the present story. You’ve got to make it sound like it’s a story being told. This is where you need to pay attention to the tense being used.
Below are links with tips to help writers decide what the most effective way to create a useful flashback would be. For those needing help with tenses, Quick and Dirty Tips is a good source.
Quick and Dirty Tips (Tenses, Tips, Explanations)
Writing Forward (When and where a flashback is useful)
Novel Writing Help (Making flashbacks natural)